safest metallicelement

cesium is so reactive  it  ignites  on contact with air. It stored in glass tubes inside inert gas or in a vacuum to  keep it safe. white  a  Mohs value of just 0.2 cesium  (cs) is soft  enough to  be cut with a butter knife. It melts at just 28 C(82F )and exposes  violently  when dropped in water. ceasium  was discovered in 1860 by the german scientists  robert bunsen and gustav  kirchhoff who isolate it  from a sample of mineral water using the recently  devekioed  technique   of flame spectroscopy silvery -gold in color cesium's most notable us in super accurate atomic clocks.

from a book Guinness world records 2018